The long-term objectives are an understanding of the development and functioning of phasic control systems which operate either automatically or under voluntary control, to affect the processing of information. Prior work has shown that modulation of the reflex blink is a means of probing the effects, both of relatively high-level automatic orienting or directed attentional processes and of a low-level "prepulse inhibition" process which depends on a transient change in stimulation occurring shortly before a reflex-eliciting probe. A specific aim of the current proposal is, first, to determine whether or not blink modulation effects which support an attentional influence early in processing could be explained, instead, as receptor-adjusting effects or as late post-perceptual effects. A receptor adjusting explanation will be tested by 1) assessing differences in attentional modulation of an early oligosynaptic and a late polysynaptic component of the cutaneous blink reflex and 2) determining whether inputs which overlap peripherally are nonetheless processed differently. A late selection explanation will be tested by determining the effects of novelty and of directing attention to a semantic category. A second aim is to investigate the relation between processing of transient stimuli and prepulse inhibition, and between both of these processes and the ability to encode stimuli. Encoding will be assessed by the size of stimulus changes which elicit dishabituation of orienting and by the time required for release from backward masking. The work will focus on early development because young infants appear to have deficient transient processing. Subjects will be young adults and fullterm and preterm infants. Heart rate deceleration will be measured to index the presence of an orienting/attentional process and reflex modulation will be measured by changes in size and latency of electromyographic (EMG) activity of m. orbicularis oculi. Future plans include extending the research to studies of scalp potentials evoked from brainstem and cortex.